Stairlifts typically comprise a carriage, mounted by way of support rollers, on a stairlift rail, for movement along the rail. The carriage includes a drive motor and gearbox and includes some form of drive transfer mechanism, such as a drive pinion or friction wheel, which cooperates with the rail to effect movement of the carriage along the rail.
In the case of curved stairlifts, the arrangement of support rollers (sometimes referred to as a skate) must be of a form which enables the carriage to accommodate and traverse bends in both a vertical plane (transition bends) and in a horizontal plane (inside/outside bends). Further, it is preferred that the skate assembly is also configured to allow the carriage to accommodate and traverse bends that combine elements of vertical and horizontal curve simultaneously. These will be referred to hereinafter as helicals. Different rails have bends of different geometry, the geometry being dictated by the form of the staircase to which the stairlift is fitted. The skate must be configured in such a manner as to accommodate all such bends, whatever their geometry and, ideally, the plane of each skate roller should be substantially parallel to the lengthwise axis of that part of the rail with which it is in contact, so as to avoid crabbing. Crabbing not only detracts from ride quality but also leads to excessive wear of the rollers and/or roller bearings. This wear, in turn, leads to increased clearance between the carriage and the rail which further detracts from ride quality.
In order to maintain the drive mechanism in the correct relationship to the rail at all times, a form of skate has been developed having a central roller set, and an outer roller set on each side of the central set. The central roller set locates the carriage on the rail and, whilst it can move along the rail, its position is otherwise fixed in relationship to the rail. The drive pinion is included in the central roller set and is thus always correctly located in relationship to the rack, extending along the rail, whatever the direction assumed by the rail.
The outer roller sets act as guide rollers, and provide stability to the carriage. In order to provide both functions simultaneously, the outer roller sets must be displaceable, simultaneously, relative to the central roller set and also provide pivotal, or steering, action relative to the central roller set.
Examples of stairlift skate of the above form can be seen in European Patent 0 853 591 and in our own published International Patent Application WO 2005/085116.
In the case of European Patent 0 853 591, the outer roller sets are carried on the outer ends of two frames which are connected by a ball and socket joint at their inner ends. Each frame is mounted on a swivel joint at a fixed point intermediate its ends. As a consequence, the movement of one outer roller set is always mirrored by the other outer roller set, and the necessary simultaneous displacement of the outer roller sets is effected by the same mechanism that provides the pivotal action to allow steering. The relatively long arcs through which the arms pivot, occupy significant space and limit the sharpness of angle through which the carriage can move. Having said that, this arrangement performs relatively satisfactorily in transition bends, where the planes in which the rollers rotate are always substantially parallel to the rail axis, but is far from ideal in inside/outside bends. Because movement in inside/outside bends is strictly mirrored, as the leading roller set enters or exits an inside/outside bend, the trailing roller set will be adjusted into an angle such that the planes in which the individual rollers rotate, will not be parallel to the rail axis. As a consequence, the rollers will crab. The same mirrored linkages also cause the outer roller sets to crab in transition bends. Further, because the axes about which the frames rotate, are spaced from the central coupling, and because a degree of backlash is inherent in mechanical linkages of this type, there is a degree of instability in the assembly, particularly when a heavy twisting load is applied to the carriage. This instability is exacerbated at steeper rail angles.
WO 2005/085116 provides an alternative. Unlike EP '591 in which movement through both transition bends and inside/outside bends is controlled by the same linkages, in WO '116, movement in transitions is accommodated by a combined rotation/displacement mechanism, whilst movement through inside/outside bends is accommodated by a linkage not dissimilar to that shown in EP '591. The outer roller sets are not mirrored together in transition bends, do not crab in transition bends, and the combined rotation displacement action in transition bends results in a mechanism which is both compact and can accommodate bends of quite an acute angle. However, in inside/outside bends, the skate described in WO '116 is susceptible to the same criticism as that made above in relation to EP '591.
It is an object of this invention to provide a stairlift, or a skate for a stairlift in which the various aspects of the invention go at least some way in addressing the problems set out above; or which will at least provide a novel and useful choice.